1. Field of the Invention
The present invention relates to a method of forming a diamond film and a film-forming apparatus, and particularly, to a method of forming a diamond film and a film-forming apparatus utilizing microwave plasma.
2. Description of the Background Art
Various methods have been invented for forming diamond from vapor phase, such as a hot-filament CVD method, a microwave plasma assisted CVD method and so forth. The microwave plasma assisted CVD method is especially suitable, among others, for forming a high-purity polycrystalline diamond film and an epitaxial diamond film, whereby a high-quality diamond film can easily be obtained compared to the case with other methods. The other methods are associated with some problems that degrade the quality of the diamond film. For example, the hot-filament CVD method involves metal contamination from filament, and a plasma jet method involves metal contamination from an electrode. Moreover, in a combustion flame method, nitrogen in the air is mixed into diamond, degrading the quality of the diamond film. Thus, the microwave plasma assisted CVD method has been widely used as a method of obtaining a high-quality diamond film, and recently, developments have been propelled for obtaining a large area of high-quality diamond film.
The microwave plasma assisted CVD method has an advantage in that such a high-quality diamond film can easily be obtained, but suffers a drawback in that the resulting film is varied in its thickness and quality in a wide range of distribution, especially when compared to the case with the hot-filament CVD method. Thus, it is particularly difficult to obtain a large size of diamond film having a uniform thickness and quality by the microwave plasma assisted CVD method. Currently, there is not even a guideline for adjusting the variation as described above, and such a guideline is still being searched for. As a guideline in forming a diamond film by the microwave plasma method, it is known to use a temperature of a substrate measured using a radiation thermometer and a thermocouple within a reactor, and further spectrum analysis by plasma emission spectroscopy or the like. However, the substrate temperature measured by the radiation thermometer is essentially associated with the plasma emission, making it difficult to obtain an accurate temperature of the substrate. Furthermore, when the thermocouple is used for a temperature measurement, the temperature cannot directly be obtained unless the substrate is in direct contact with the thermocouple. Even if the direct contact was possible, such contact would cause disturbance, which affects formation of the diamond film. Whereas, when the plasma emission spectroscopy is used for diagnosing a plasma state, observation on the spot is possible without any contact, causing no disturbance to the plasma state. Thus, conventionally, the diagnosis using the plasma emission spectroscopy has been actively performed. The measurement using the plasma emission spectroscopy has been successful in certain ways, for instance, contamination by nitrogen, which significantly interferes with the formation of the diamond film, can be found instantly. However, the plasma emission spectroscopy has not yet reached the level where the quality and the deposition rate of the diamond film can be predicted.
The present invention is directed to provide a method for forming a diamond film from reaction gas excited by microwave, and particularly for forming a large size of a high-quality diamond film by controlling a manufacturing condition based on information on spectroscopic measurement of plasma emission, and to provide a film-forming apparatus for forming such a diamond film.
In the method of forming a diamond film according to the present invention, a gas mixture of hydrocarbon gas and hydrogen gas is introduced into a reactor where the gas mixture is excited by microwave which is also introduced into the reactor to generate plasma, in order to form a diamond film on a substrate. In the forming method, plasma emission is spectroscopically measured to control a formation condition of the diamond film such that the spectrum of a carbon molecule (C2: hereinafter referred to as xe2x80x9ccarbon moleculexe2x80x9d) falls within a predetermined range of requirement.
The formation method of the diamond film from the microwave-excited plasma of this invention is based on a new idea in that the spectrum of the carbon molecule is strongly correlated with the quality of the diamond film and its distribution, and hence only an emission spectrum band of the carbon molecule is herein observed. Thus, a controlling method can be simplified, since the only requirement is to adjust the formation condition of the diamond film such that the spectrum of the carbon molecule is within the predetermined range, and therefore the formation condition of the diamond film can be precisely adjusted. As a result, the distribution of quality, i.e. spatial variation of quality, is suppressed, so that a large area of homogeneous and high-quality diamond film can be obtained. Any apparatus may be employed for forming the diamond film described above, in which reaction gas is excited by microwave to attain a plasma state and the diamond film is formed on a substrate by the plasma. A microwave plasma assisted CVD apparatus or another apparatus may be used.
In the method of forming a diamond film according to the present invention, the spectrum of a carbon molecule is a vibration spectrum of the carbon molecule, and a formation condition is controlled such that a vibration temperature obtained by such a spectrum falls within a predetermined range.
The inventors of the present invention spectroscopically measured the emission of microwave plasma, and found that the vibration temperature of a carbon molecule can be derived from the emission spectrum band of the carbon molecule, i.e. one of activated molecule species constituting the plasma. They also came to a new idea in that the vibration temperature is closely related to the deposition rate and quality of the diamond, and the distribution thereof. The vibration temperature of the carbon molecule can be derived using the procedure described below.
In plasma, electrons are much lighter than atomic nuclei, and hence moves much faster. This allows the movement of the electrons and that of the atomic nuclei to be precisely separated for further discussion. Such a way of discussing these movements independent of each other is a precise approximation method called Born-Oppenheimer approximation. When Born-Oppenheimer approximation is possible, the intensity Ievxe2x80x2vxe2x80x3Jxe2x80x2Jxe2x80x3 of spectral lines contained in a band spectrum emitted due to the transition of a molecule between electron states can be represented by the equation (1) below.
Ievxe2x80x2vxe2x80x3Jxe2x80x2Jxe2x80x3=Cf4qvxe2x80x2vxe2x80x3SJxe2x80x2Jxe2x80x3xc3x97exp[[xe2x88x92(hc/kTex) Te]+[xe2x88x92(hc/kTvib) G (vxe2x80x2)]+[xe2x88x92(hc/kTrot) F (Jxe2x80x2)]]xe2x80x83xe2x80x83(1) 
wherein e is a type of electron-term transition, v is the quantum number of vibration, J is the quantum number of rotation, and an addition of xe2x80x2 indicates a high level whereas that of xe2x80x3 indicates a low level. Moreover, C is a constant, f is the vibration number of the spectral lines, qvxe2x80x2vxe2x80x3 is a Franck-Condon factor and SJxe2x80x2Jxe2x80x3 is a Honl-London factor. Furthermore, h is a Planck constant, c is the speed of light, and k is a Boltzmann constant. In addition, Tex, Tvib and Trot indicate an excitation temperature, a vibration temperature and a rotation temperature, respectively, and Te, G(vxe2x80x2) and F(Jxe2x80x2) indicate the term values in the electron state, in the vibration state and in the rotation state, respectively. Noting the transition between certain electron states, the equation (1) is separated by the term of the vibration temperature and that of the rotation temperature. When qvxe2x80x2vxe2x80x3 and SJxe2x80x2Jxe2x80x3 are known and the rotation spectrum can be resolved for measurement, the vibration temperature and the rotation temperature can be obtained independently of each other. When the rotation spectrum cannot be resolved due to e.g. limitation of wavelength resolution of a spectroscope, if qvxe2x80x2vxe2x80x3 is known, the vibration temperature Tvib can be obtained from the intensity of a band head (Jxe2x80x2=Jxe2x80x3=0). Therefore, the equation (1) can be rewritten as the equation (2) below.
Ivxe2x80x2vxe2x80x3=C1f4qvxe2x80x2vxe2x80x3exp[xe2x88x92(hc/kTvib) G (vxe2x80x2)]xe2x80x83xe2x80x83(2) 
wherein C1 is a constant independent of f. The intensity Ivxe2x80x2vxe2x80x3 of the spectral lines and the wavelength G (vxe2x80x2) are directly obtained by the plasma spectroscopic measurement, so that the equation (2) can further be rewritten as the equation (3) below.
In [Ivxe2x80x2vxe2x80x3/f4qvxe2x80x2vxe2x80x3]=C2xe2x88x92(Evxe2x80x2/kTvib)xe2x80x83xe2x80x83(3) 
From the equation (3), In [Ivxe2x80x2vxe2x80x3/f4qvxe2x80x2vxe2x80x3] is plotted with respect to Evxe2x80x2, and the inclination is obtained by fitting, to further obtain the vibration temperature of the molecule. Here, C2 is a constant independent of f, and Evxe2x80x2 indicates vibration energy.
According to the method described above, the wavelength resolution required for the spectroscope may be at a relatively low level of 0.3 nm. Therefore, plasma can easily be estimated by an inexpensive apparatus. The vibration temperature of C2 molecule obtained as described above is close to the equilibrium with gas temperature determined by the kinetic energy of other activated species gas or neutral gas in the reactor, so that it can be approximately estimated as plasma gas temperature. The gas temperature of plasma is closely related to the film deposition rate and quality of diamond, and the distribution thereof. As described above, the present invention is based on a new idea in that the film deposition rate or the quality of diamond can be estimated by the vibration temperature of the carbon molecule that can easily be obtained. The vibration temperature of the carbon molecule can readily be controlled by changing the power of inputting microwave, pressure, gas flow rate or the like.
In the method of forming a diamond film according to the present invention, a formation condition is controlled such that the vibration temperature of the carbon molecule falls within the range between 2000 and 2800 K.
By controlling the vibration temperature to be within the range described above, a high-quality diamond film can rapidly be formed. For example, if the diamond film is formed with a vibration temperature within a range between 2400 and 2700 K, a diamond film transparent from ultraviolet to infrared regions can be obtained. Moreover, if the diamond film is formed with a vibration temperature within a range between 2200 and 2800 K, a diamond film with thermal conductivity of 1000 W/mK, which is applicable to a heat sink or the like, can be obtained. If the vibration temperature is less than 2000 K, the film deposition rate is lowered, degrading crystallinity of a resulting diamond film. In addition, distribution of the quality such as crystallinity may be varied in certain locations. On the other hand, if the vibration temperature exceeds 2800 K, the film deposition rate is increased, which now makes the crystallinity of the resulting diamond incomplete while increasing the positional variation in quality.
In the method of forming a diamond film according to the present invention, at least one of microwave-inputting power, pressure in the reactor and flow rate of each reaction gas in the formation condition of the diamond film is controlled such that the spectrum falls within a predetermined range.
The formation condition as described above can easily be controlled artificially, and control of at least one such condition allows the vibration temperature to be in the predetermined range, and hence a large area of high-quality diamond film can be obtained.
Moreover, in the method of forming a diamond film according to the present invention, the vibration temperature of the carbon molecule can be obtained from a spectrum band having a difference of +1 or xe2x88x921 between a high vibration level and a low vibration level.
Though no selection rule exists in C2 molecule for transition between the vibration levels, the vibration temperature can precisely be obtained from the transition with a difference of xc2x11 between the levels, because such transition occurs with a probability higher than transition with a difference of other value between levels. However, the level difference in vibration is not necessarily xc2x11, and the level difference of 0 may also be used.
In the method of forming a diamond film according to the present invention, an emission spectrum band of a carbon molecule within a range between the wavelengths of 465 and 475 nm is used to obtain a vibration temperature.
The emission spectrum band of the carbon molecule in this wavelength range has a vibration level difference of +1 and has a particularly high probability of transition, so that the vibration temperature can be obtained with high precision. Furthermore, even when automatic control is employed, obvious peaks can be seen, and the ratio of the peak intensities in the above-described wavelength range may be obtained to simplify the automatic control.
The film-forming apparatus of a diamond film according to the present invention includes a reactor in which reaction gas is excited to generate plasma; a microwave generating apparatus generating microwave; a spectroscope analyzing a spectrum of light emitted from the plasma; and an arithmetic unit obtaining a vibration temperature from an emission spectrum band of a carbon molecule obtained by the spectroscope.
The arrangement described above facilitates obtainment of the vibration temperature from the emission spectrum band of the carbon molecule. Though the arithmetic unit is preferably a microcomputer into which a software is installed, it may also be a wired logic circuit. In the arithmetic operation, the peaks in the emission spectrum band of the carbon molecule correspond to certain wavelengths, and thus the vibration temperature can rapidly be obtained by taking a ratio of the peak intensities in the vicinity of such wavelengths.
The film-forming apparatus according to the present invention further includes a control unit for controlling at least one factor of microwave-inputting power, pressure within a reactor and flow rate of reaction gas, such that the vibration temperature falls within a predetermined range, based on the value of the vibration temperature obtained by the arithmetic unit.
By the above-described arrangement, a large area of a high-quality diamond film can be obtained by automatically controlling a formation condition of the diamond film. The automatic control is desirably performed such that the diamond film is formed after the vibration temperature of a carbon molecule in plasma is brought to be within the predetermined temperature.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.